Steerable phased array antennas usually require the transfer of array energy between a multiplicity of antenna elements, often several thousand in number, each of which has an associated phase shifter with a transmitter and/or a receiver. The conventional approach for distributing this energy has been a corporate-fed array.
FIG. 1 shows a corporate-fed array 20. A corporate feed or corporate distribution network 21 comprises a network of power dividers and series-parallel transmission lines and drives a plurality of electronics modules 22. These electronic modules 22 comprise pairs of phase shifters 23 and amplifiers 24. The electronic modules 22 drive an array of antenna elements 25, such as dipoles. When the phase shifters 23 in the electronic modules 22 are adjusted so that the antenna elements 25 are driven in a linear phase progression, the array of antenna elements produces equiphase fronts, which travel at an angle to the array. This results in a concentrated beam of energy in a direction perpendicular to the equiphase fronts. The direction of this concentrated beam can be changed in a predictable manner by changing the settings of individual phase shifters 23 to new predictable settings. In this manner, the array of antenna elements 25 can be used in conjunction with the electronics modules 22 to sweep a composite beam of radiated energy across a field of view.
The corporate-fed array 20 has several limitations including high transmission line losses at high frequencies and the need for attenuators or special couplers in series with the transmission lines to provide a tapered aperture distribution, that is, individual electronics modules 22 may need to be coupled to the corporate feed network 21 with different values of coupling so that a specified tapered amplitude distribution across the array is provided. Such amplitude distributions are required when low sidelobes are specified in resulting antenna patterns. These two limitations reduce efficiency of the array. Conventionally, several stages of amplification have been added to each electronics module 22 to compensate for these limitations. However, these added stages of amplification increase complexity, power requirements, phase and amplitude errors, and cost. The increased complexity also reduces reliability and, in the case of monolithic integrated circuits, reduces yield. Another approach for distributing array energy, which avoids the limitations of the corporate-fed array, is a space-fed array.
FIG. 2 shows a space-fed array 26. A simple feed horn 27 distributes energy to all antenna elements in an array by illuminating the back side of the array. Each antenna element 25 on the face of the array has a corresponding antenna element 28 that faces the feed horn 27 to receive this energy. Thus, in this approach, each electronics module 22 comprises two antenna elements 25 and 28, a phase shifter 23, and an amplifier 24.
The horn illumination pattern produced with this approach provides the varied coupling to the electronics modules 22 and, therefore, the tapered amplitude distribution across the aperture required for low sidelobes. Also, with this approach, transmission through free-space is much less lossy than through any other high frequency transmission line medium. Thus, fewer stages of amplification are required in each electronics module 22 for the space-fed array 26 than for the corporate-fed array 20. In addition, space-feeding randomizes phases of signals in the antenna elements 28 thereby reducing the probability of high quantization sidelobe levels in the antenna pattern, which are caused by digital phase shifting. Digital phase shifting is the most common phase shifting method embodied in phased array antennas. However, the principal disadvantage of a space-fed array 26 is the spatial distance between the feed horn 27 and the array and thus the resulting physical thickness of the array assembly. Typically, this spatial distance is equal to half the array diameter. This disadvantage has been eliminated by using a radial line distribution network, or flat plate-fed array.
FIG. 3 shows a flat plate-fed array 29. A flat plate-fed array 29 is essentially a special type of space-fed array in which feed-point spacing is reduced to about one-half wave-length and feed energy is guided radially outward between two flat plates 30 and 31 which act as a radial waveguide. See for instance, U.S. Pat. No. 3,576,579 to Appelbaum et al. As shown in FIG. 3b, taken from an embodiment of Appelbaum et al., a multimode launcher 32 generates a sum mode .epsilon., an azimuth difference mode .DELTA.A and an elevation difference mode .DELTA.E and feeds them into the radial power divider 33. This multimode launcher 32, which can also be used with space-fed arrays, provides an amplitude monopulse capability. Wave energy decreases in amplitude as distance increases from the feed-point. The radial power divider 33 comprises a multiplicity of directional couplers, distributed about concentric circles in the radial waveguide, which pick up that wave energy and transfer the energy to an array 34 of phase shifters and antenna elements. The directional couplers replace the pickup antenna elements 28 on the inside face of the space-fed array 26 of FIG. 2. Tapered amplitude distribution is achieved by adjusting coupling values in each concentric ring.
The flat plate-fed array 29 has several limitations. The array of antenna elements 25, fed by the flat plate, comprises concentric rings, so each ring of antenna elements requires a different coupler design. These different couplers must be indexed circumferentially, i.e., their physical configuration must be radially symmetric, to couple to a radially propagating wave. However, except in a circularly polarized array, the antenna elements 25 must all be aligned parallel, vertically, or horizontally, for instance. Ease of assembly, or electrical connections, for instance, may require a fixed orientation of antenna elements 25 even in a circular polarized array. As a result, there are no more than two antenna element modules with a common design in each ring of antenna elements 25. These couplers must be manufactured and assembled in the array extremely accurately for high microwave and millimeter wave frequency applications. Small tolerance errors perturb the required aperture distributions and may impose practical limits on achieving low sidelobe levels. Additionally, the cost of manufacturing EHF couplers, assembling them in an array, and performance verification testing is high.
FIG. 3c shows a section view of a modification of the FIG. 3b embodiment. A single ring of directional couplers 35 is used at the periphery of a circular radial waveguide. The energy from each directional coupler 35 is distributed to a set of antenna elements 25 through a stripline power divider (not shown) where power division values are tailored to match the required amplitude distribution of the array. This approach suffers from many of the same disadvantages as both the space-fed approach and the corporate feed approach.
FIG. 4 shows another radial waveguide 37 approach. Coaxial line pickup probes 38 replace directional couplers. Amplitude distribution is controlled by varying the spacing between the walls of the radial waveguide 37. Although this eliminates the need to index the pickup probes 38 circumferentially, mutual coupling between probes 38 is extremely sensitive to manufacturing and assembly tolerances, and is very frequency dependent. These factors impose a narrow frequency band limitation on this approach. Furthermore, coaxial lines, which would connect to the pickup probes 38, are lossy at EHF frequencies.
FIG. 5a shows a known distribution network 39 for a slotted waveguide array antenna. The antenna consists of a planar array of radiating slots 40 distributed along a coplanar wall 41 in each of an ensemble 42 of parallel waveguides. The distribution network 39 comprises a waveguide ensemble 42 fed or excited by an orthogonal excitation waveguide 43 or waveguides, through a row of inclined exciting slots 44 in a wall 45 common to the excitation waveguide 43 and the parallel waveguide ensemble 42, one slot per waveguide. A predetermined amplitude distribution is achieved in the plane parallel to the axis of the exciting waveguide 43 by adjusting the tilt angle of each inclined exciting slot 44, and in the orthogonal plane by adjusting the displacement of the radiating slots 40 from the center line of the waveguides as well as slot width and length. The waveguide can include a tapered waveguide load at the end of each waveguide.
FIG. 5b illustrates slot 40 and 44 configurations in a typical quadrant of a slotted waveguide array antenna having a circular aperture 46. FIG. 5c illustrates a millimeter wave, center-fed slotted waveguide array distribution network 47. This network 47 propagates radiation to each of four quadrants, similar to the quadrant of FIG. 5b. This distribution network 47 has monopulse capability. FIG. 5d shows a schematic diagram of a monopulse comparator network, which is used with a slotted, waveguide array distribution network. FIGS. 5b and 5c illustrate well known slot array antenna technology, and have been described in the "Microwave Journal" Magazine, July, 1985. FIGS. 5a and 5b have been described in the "Microwave Journal" Magazine, June, 1988.
FIG. 5e shows a rectangular waveguide 48 with the same dimensions as the waveguides of FIG. 5a having a rotated series slot 49 and a longitudinal shunt slot 50. This figure is used to illustrate how coupling values are computed in a slotted waveguide array distribution network. For example, FIG. 5f shows the parameters and equivalent circuit of a rotated series slot 49 while FIG. 5g shows the parameters and equivalent circuit of a longitudinal shunt slot 50. The ratio of input impedance to output impedance of the rotated series slot is a function of the angle of the slot 49 relative to the waveguide. The ratio of input conductance to output conductance of the longitudinal shunt slot 50 equals: ##EQU1## where K is a function of frequency and waveguide dimensions and is well known, "a" is height of the waveguide and "d" is a distance between the slot 50 and center of the height of the waveguide, known as centerline off-set. Such coupling slots in waveguides are well known, as are other slot configurations that could be used in such slotted waveguide array antennas and distribution networks.
Since no phase shifters, such as 23 of FIG. 1, are contained in a slotted waveguide array antenna, its radiation beam pattern has a fixed angular orientation. Beam scanning can only be achieved mechanically, that is, by physically reorienting the antenna, or by changing frequency. Mechanical scanning is slow compared to scanning achieved by electronically adjusting phase shifters and requires more space to implement. A consequence of the latter is that such antennas cannot both scan and remain conformal to a surface, such as the skin of an aircraft. Slotted waveguide array antennas can also scan their beam pattern by changing frequency, but this method is incompatible with their use in communication systems and is frequently undesireable in other applications such as radar.